After the update to Smack 4 Spark is not able to receive files and doesn't show File transfer and Screenshot sending button on the toolbar in the chat window. There is also no File transfer context menu for the contact.
I believe that this is a timing issue. A file transfer manager object gets initialized when the connection is authenticated. With Smack 4, authentication appears to have finished even before the transfer manager object was created in the first place. This caused the object to go uninitialized.
I've modified the code in such a way that initialization need not depend on authentication.